What is Xylem?
Xylem is the tree's water-transport system and is what we commonly call "wood." It carries water and dissolved minerals from the roots up to the leaves — sometimes over 60 meters in tall tropical emergents.
How It Works
Key Components
- Vessel elements: Wide, open tubes that carry large volumes of water quickly (found in hardwoods).
- Tracheids: Narrow, elongated cells that transport water more slowly (dominant in softwoods/conifers).
- Xylem parenchyma: Living storage cells within the wood.
- Xylem fibers: Thick-walled cells that provide structural strength.
Transport Mechanism
Water moves upward through the xylem by transpiration pull — evaporation from leaves creates negative pressure that draws water up through the continuous water columns. Cohesion between water molecules keeps the columns intact.
Sapwood vs. Heartwood
- Sapwood: Outer xylem that actively transports water — lighter in color.
- Heartwood: Inner xylem that no longer transports water — filled with extractives, often darker and more durable.
Costa Rican Examples
Teca (Tectona grandis)
Large vessels produce efficient water transport, contributing to its fast growth in plantations.
Cocobolo (Dalbergia retusa)
Dense xylem with abundant extractives creates the spectacularly colored, extremely hard heartwood prized worldwide.
Why It Matters
- Timber properties: Xylem structure determines wood density, hardness, and workability.
- Tree physiology: Xylem damage from drought or disease can cause branch dieback.
- Identification: Vessel patterns visible under magnification help identify wood species.